The subject matter disclosed herein relates to airflow control and, more particularly, to an aircraft airflow system that operates during a power failure condition.
Bleed systems on aircraft generally involve taking air from the aircraft engine, and regulating it down to a usable temperature and pressure. Pressure is usually regulated through valves, such as butterfly valves being opened certain amounts from zero to ninety degrees, to decrease or increase pressure downstream of the valve. The valves are generally either a proportional valve or an integrating valve, and can be controlled either pneumatically or electronically.
Pneumatic control is done through physical components, flow passages, levers, etc. Electronic controls control the valve through electrical signals. In particular, an electronically controlled proportional valve is generally operated by receiving an electrical control signal which corresponds to a valve position. An electronically controlled integrating valve is controlled with an electrical signal that corresponds to a valve velocity, causing the valve to open or close due to the valve travelling at a velocity for a certain amount of time.
It is desirable to maintain stable pressures in the bleed system to improve performance and decrease wear on the system. This includes resisting cycling and input disturbances in the system. Cycling is when pressure values downstream of the valve cycle throughout a range of pressures, to average out to the desired pressure. For example, if the desired pressure is 45 psig, but it is cycling from 40 psig to 50 psig to get an average of 45 psig, that cycling creates a lot of extra wear on system components from the constant fluctuations. The cycling can be due in part to frictional forces that must be overcome to open or close the valve. Overcoming the frictional forces can result in a backlash of force due to the larger amount of force needed to overcome the initial frictional forces to initiate valve movement. After the initial frictional forces are overcome, the valve can move very rapidly, which can turn into cycling if movement is too rapid and the desired target is overshot. Input disturbances (which can initiate cycling) come from things such as a change in throttle which causes a power change in the engine. Throttling up the engine can cause the pressure to quickly and dramatically change. The bleed system then responds to this rapid change, trying to regulate the pressure to a stable, usable level once again.
In some systems, valves are configured to fail to either a fully open or fully closed state upon loss of power. This results in an inability to accurately regulate system supply pressure in some instances.